1. Field of the Invention
The present invention relates to an electrically operated power steering apparatus for applying the power of an electric motor as an assistive steering force to a mechanical steering system to reduce the manual force required to steer a motor vehicle which incorporates such an electrically operated power steering apparatus, and more particularly to an electrically operated power steering apparatus which produces a large assistive steering force while the motor vehicle is running at a low speed and improves the response of the assistive steering force while the motor vehicle is running at a high speed.
2. Description of the Related Art
There has been known an electrically operated power steering apparatus for establishing a target current to energize an electric motor based on a steering torque detected by a steering torque sensor and a vehicle speed detected by a vehicle speed sensor. The electrically operated power steering apparatus includes a PI (proportional plus integral) controller or a PID (proportional plus integral plus derivative) controller for compensating for a phase delay and a difference between the target current and a motor current which actually flows through the electric motor.
One conventional electrically operated power steering apparatus is disclosed in Japanese laid-open utility model publication No. 3-118173.
FIG. 3 of the accompanying drawings shows a conventional electrically operated power steering apparatus, and FIG. 4 of the accompanying drawings shows a controller of the electrically operated power steering apparatus shown in FIG. 3.
As shown in FIG. 3, the electrically operated power steering apparatus, generally denoted at 1, includes a steering wheel 2, a steering shaft 3 coupled to the steering wheel 2, hypoid gears 4, a rack and pinion mechanism 5 comprising a pinion 5a mounted on the steering shaft 3 and a rack shaft 5b having rack teeth held in mesh with the pinion 5a, a pair of tie rods 6 (one shown) coupled to respective opposite ends of the rack shaft 5b, a pair of front wheels 7 (one shown) operatively connected to the respective tie rods 6, and an electric motor 8 having a drive shaft operatively coupled to the steering shaft 3 through the hypoid gears 4. The steering wheel 2, the steering shaft 3, the rack and pinion mechanism 5, the tie rods 6, and the front wheels 7 jointly serve as a steering mechanism.
The electrically operated power steering apparatus 1 also has a steering torque sensor 10 mounted on the steering shaft 3 for detecting a steering torque applied to the steering wheel 2 and outputting an electric steering torque signal T converted from the detected steering torque, a vehicle speed sensor 11 for detecting a vehicle speed of the motor vehicle and outputting an electric vehicle speed signal V converted from the detected vehicle speed, and a controller 12 for controlling operation of the electric motor 8 based on the steering torque signal T and the vehicle speed signal V which are supplied from the steering torque sensor 10 and the vehicle speed sensor 11, respectively.
When the steering wheel 2 is manually turned by the driver of the motor vehicle, the steering torque sensor 10 detects an applied steering torque, converts the detected steering torque into a steering torque signal T, and sends the steering torque signal T to the controller 12.
Upon rotation of the steering shaft 3, the rotation of the pinion 5a is converted to axial linear motion of the rack shaft 5b by the rack and pinion mechanism 5, causing the tie rods 6 to steer the front wheels 7.
The vehicle speed sensor 11 detects a vehicle speed of the motor vehicle, converts the detected vehicle speed into a vehicle speed signal V, and sends the vehicle speed signal V to the controller 12.
Based on the steering torque signal T and the vehicle speed signal V which are supplied from the steering torque sensor 10 and the vehicle speed sensor 11, respectively, the controller 12 generates a motor current I.sub.M to energize the electric motor 8 therewith. When the motor current I.sub.M is supplied to the electric motor 8, the electric motor 8 is energized to apply an assistive steering force through the hypoid gears 4 to the steering mechanism for thereby reducing the manual steering force which is applied to the steering wheel 2 by the driver.
As shown in FIG. 4, the controller 12 includes a target current setting means 13 for converting the steering torque signal T and the vehicle speed signal V into a target current I.sub.MS according to Table 1 (see FIG. 6) of data representing steering torque signals T corresponding to target currents I.sub.MS with vehicle speed signals V used as a parameter, a subtractor 14 for calculating a difference or error .DELTA.I.sub.M between the target current I.sub.MS and the motor current I.sub.M, a PI control means 15 for correcting the difference .DELTA.I.sub.M in a proportional plus integral control mode to produce a motor current signal I.sub.o, and a motor driving means 16 for generating a motor current I.sub.M based on the motor current signal I.sub.o and supplying the generated motor current I.sub.M to energize the electric motor 8.
The PI control means 15 has a proportional element and an integral element arranged parallel to each other, and has a transfer function F(j.omega.) whose gain G and phase angle .theta. are shown in the Bode plot of FIG. 5 of the accompanying drawings.
In FIG. 5, it is known that in a range where the angular frequency .omega. corresponding to a steering rotational speed is low, there is a phase delay (.theta.=-90.degree.), but the gain G (20logG) is greatly improved, and that in a range where the angular frequency .omega. is high, the gain G (20logG) is low, but the phase delay is greatly improved.
As can be seen from FIG. 5, the phase delay is -90.degree. in a range A of angular frequencies .omega. where the gain G (20logG) is large. Since the motor current I.sub.M is large if the target current I.sub.MS is large (V:L) while the motor vehicle is running at a low speed as indicated by Table 1 shown in FIG. 6, an actual steering assistive torque Ta produced by the electric motor 8 which is represented by the equation (1), given below, can be expressed by the equation (3), given below, because of a condition indicated by the inequality (2), given below. EQU Ta=k.sub.T *I.sub.M -J.sub.M *.theta..sub.M" -C.sub.M *.theta..sub.M' .+-.f,(1) EQU k.sub.T *I.sub.M &gt;&gt;J.sub.M *.theta..sub.M" +C.sub.M *.theta..sub.M' .+-.f,(2) EQU Ta.apprxeq.k.sub.T *I.sub.M ( 8)
where k.sub.T is the torque constant of the electric motor, J.sub.M the moment of inertia of the electric motor, C.sub.M the viscosity coefficient of the electric motor, f the friction of the electric motor, .theta..sub.M" the steering rotational angular acceleration, and .theta..sub.M' the steering rotational angular velocity.
Therefore, if the target current I.sub.MS is large and the motor current I.sub.M is large while the while the motor vehicle is running at a low speed, then the steering feel which the driver has is not adversely affected even when the difference .DELTA.I.sub.M between the target current I.sub.MS and the motor current I.sub.M becomes a phase difference of 90.degree..
If target current I.sub.MS in Table 1 shown in FIG. 6 is small while the motor vehicle is running at a high speed, then since the motor current I.sub.M is also small, the effect of the right-hand side of the inequality (2) is so large that it cannot be ignored.
Therefore, when the current control signal I.sub.o is produced while the difference .DELTA.I.sub.M between the target current I.sub.MS and the motor current I.sub.M is equal to a phase difference of 90.degree., the phase difference represented by the right-hand side of the inequality (2) is added, resulting in a degradation of the steering feel.